Process for the passivating anodization of copper in a medium of molten fluorides, and use for the protection of copper parts of fluorine electrolysers

ABSTRACT

A process for producing a strong, adherent protective layer on copper parts, with a high rate of covering of the substrate, by passivating anodization, characterized in that the copper parts are immersed in a liquid KF, 2HF bath and subjected to anodic current of low surface-related density which is less than 0.1 A/dm 2 , which current may be continuous or intermittent.

TECHNICAL FIELD

The present invention concerns a process for the passivating anodisationof copper parts in a medium of molten fluorides forming an adherentprotective layer with a high covering rate; the process can be used inparticular but not exclusively for the protection of the copper partsused in electrolysers for the production of fluorine.

STATE OF THE ART

The process for producing fluorine by electrolysis uses a bath of moltenfluorides, which is generally a mixture of hydrogen fluoride andfluorides of alkali metals and/or ammonium. The anodes of carbonaceousmaterial are immersed vertically in the bath and are supplied withelectrical current by current supply members which are usually ofcopper. The copper-anode junction which represents a weak point usuallyoccurs at the top of the anode, in which case the copper current supplymember and the copper-anode junction are partially immersed in the bathand are subjected to the action of the bath and the fluorine bubbleswhich are given off at the anode. Passivation of the copper occurs onthe one hand by virtue of immersion in the bath of liquid fluorides andon the other hand due to anodisation when the electrolysis cell is putunder voltage, but the properties of the layer obtained are highlyunsatisfactory for providing effective protection for the copper. Thecopper is thus dissolved, resulting in a slow, regular deterioration inthe copper-anode contact, which requires the electrolysis cell to bestopped and renovated, in particular requiring the current supplymembers to be restored and the anode changed. That renovation operationis effected approximately once per year.

The copper-anode junction may also advantageously be made at the bottom.In that case the copper current supply members pass through the totalthickness of the bath before being connected to the base portions of theanodes. It is then necessary for them to be insulated, to preventdissolution thereof; it is possible for example to provide sheaths whichare capable of resisting the bath. An arrangement of that kind isdescribed in SU-patent No. 193 454 which describes sheathing for thecurrent supply members which is effected by means of magnesium, andprotection for the copper-anode contacts by means of a chemically inertinsulating agent (fluorinated hydrocarbon). The use of such protectionarrangements is a delicate matter and they involve the use of productswhich give rise to problems.

However, the document `Electrodeposition and surface treatment` I(3)--1973, pages 256-265 (Battelle) discloses a treatment for the anodicpassivation of copper in a liquid KF-HF bath. To form the passivatinglayer, that document describes a constant anodic passivation current ofat least 0.4 A/dm² in an equimolecular KF-HF bath at 245° C., the timefor which that current is applied decreasing in proportion to anincreasing current; for a passivation period of greater than about 60minutes, it is noted that the value of the passivation current is stillbetween 0.4 and 0.45 A/dm² (FIG. 2), in other words 0.4 A/dm² representsa minimum asymptotic value of the passivation current.

That document also discloses anodic passivation of the copper in ananhydrous HF bath at 20° C., in which case the minimum asymptotic valueof the anodisation current is about 0.15 A/dm².

The difficulty involved in significantly reducing corrosion of thecopper and preventing deterioration in the copper-anode contacts inliquid KF-x HF baths (throughout the description the expression KF-x HFwill means a mixture in which the number of moles of HF is exclusivelyequal to or close to 2), for the electrolytic production of fluorine,constitutes at the present time a limit on improving and developingfluorine electrolysers with higher levels of performance.

OBJECT OF THE INVENTION

Thus the applicants continued their research, the main object of whichis to provide for durable and effective passivation of copper in a bathof liquid fluorides by means of a process which is simple to carry intoeffect. In particular the passivation operation is to provide durableand effective protection for the copper under the conditions encounteredin the electrolytic production of fluorine; in particular it mustwithstand the action of the electrolysis KF, xHF baths, the fluorineproduced and the electrolysis current.

Another object is the controlled production or manufacture of a layerfor protecting copper in a medium of molten fluorides, which isfluid-tight and which has a high level of adhesion to the coppersubstrate and a high rate of covering the substrate.

Another object is to produce an electrically insulating layer.

A further object is to produce a layer which is thin while, by virtue ofthe strong cohesion of the particles which constitute the layer, it hasgood mechanical characteristics, in particular resistance to abrasion,wear, impacts... .

A further object of the invention is to use an electrochemical processwhich makes it possible to effect the passivation operation in tanks andon the production site, opening the production of such tanks.

Another object is to avoid slow dissolution of the copper anddegradation of the copper-anode junctions during electrolysis of liquidfluoride baths and in particular the KF, xHF bath.

DESCRIPTION OF THE INVENTION

The invention is a process for the passivating anodisation of copperparts in a liquid KF, xHF medium (x close to 2) which makes it possibleto produce a mechanically and electrically strong adherent protectivelayer with a high rate of covering of the copper substrate,characterised in that said copper parts, once immersed in the liquid KF,xHF bath, are subjected to an anodic current of low surface-relateddensity, calculated with respect to the immersed copper surface area, ofless than 0.1 A/dm². That treatment is applied for a variable period oftime which is always greater than a limit value which is dependent onthe value of the anodic current density.

The bath is formed by a liquid KF, xHF mixture in which the amount of HFis preferably between 38 and 42.5%; that mixture is usually employed asa bath for the electrolytic production of fluorine.

The bath is to be liquid; it is advantageous to operate under conditions(temperature and concentration) such that the vapour pressure of HF doesnot exceed 50 mm of mercury, or that there is not more than 7% (byweight) of HF which is entrained by the gases. Thus it is advantageousto operate at a temperature of between 85 and 105° C.

For that type of bath which is used in the electrolytic production offluorine, the applicants researched a process for the passivation ofcopper by anodisation, which process is to be such that the protectivelayer formed is resistant both to the action of the bath which is acid(presence of 2 HF) and the action of the fluorine which is given off inthe course of the electrolysis operation. Such a bath is essentiallydifferent from those described by Battelle which are (i) one which isvery basic, taking account of the presence of a single HF molecule whichis bonded to the KF molecule, dissociation giving the species F⁻ and HF₂⁻ (ii) the other being free of KF. In such baths the activity of theconstituents is different from that encountered is the baths used in theinvention and the temperatures described are also very differenttherein.

It follows that Battelle describes anodisation current strengths whichare higher than a floor value (for example 0.4 A/dm²) which is itselfgreatly higher than the maximum strength prescribed by the applicants.

Accordingly the conditions for formation (in particular nucleation andgrowth...) of the passivating layer, as described by Battelle, are verydifferent and produce a layer with properties such as homogeneity ofdensity of adhesion, which are also highly different. Those operatingconditions thus cannot be used to provide the conditions for theformation of a protective layer in a KF, xHF medium, which complies withthe applicants' requirements, which layer must be capable ofwithstanding the bath, the fluorine which is given off and theelectrical conditions in the electrolysis operation, and it also to beadherent, compact and solid in the course of time.

In accordance with the invention a dc voltage is applied between thecopper part to be protected and a cathode of any conductive material,for example steel, which is also immersed in the bath. That voltage andalso the shape, positioning, spacing etc of the cathode are such thatthe current density at all points of the surface to be protected isuniform and is maintained at a low value.

The low current density applied to the surface to be protected may bemaintained at a constant value in dependence on time and throughout theentire duration of the treatment, in which case the anodisationtreatment is referred to as being a constant-mode treatment; it may alsobe of a variable value in which case the treatment is referred to as avariable-mode treatment.

It is an interesting proposition to use the lowest possible levels ofcurrent density; in fact, for low values of current density, thesubstrate covering rate and the compactness of the protective layer arebetter. Moreover the quality of the protective layer produced by theanodic treatment improves in proportion to increasing length of theperiod of treatment.

However, with excessively low levels of current density, the duration ofthe treatment increases exponentially and becomes prohibitive; likewisefor a given level of current density the quality of the protective layerformed remains practically the same when the duration of the treatmentis prolonged to an exaggerated extent. Thus the current density mustgenerally be less than 0.1 A/dm² but preferably less than 0.5 A/dm² andmore particularly less than 0.025 A/dm². As regards the duration of thetreatment, practically but not limitatively, it does not exceed 20 hoursand preferably 15 hours and consequently the process avoids using, in aconstant-mode treatment, a current density which is less than 0.01A/dm².

For levels of current density at the upper limit of 0.1 A/dm², thetreatment time is generally more/than 0.5 hour but for levels of currentdensity of the order of 0.05 A/dm², the usual practice is to employtreatment times of between 2 and 4 hours.

The curve shown in FIG. 1 gives an illustration of a possiblerelationship between the current density (shown in ordinates) and thetreatment time (shown in abscissae) for producing the same protectivelayer when the current density (or strength) is kept constant in thecourse of the treatment, for a bath KF, xHF, containing 40.5% by weightof HF.

In an advantageous embodiment of the invention (variable mode), thecurrent density applied is variable in dependence on time, whileremaining within the above-described limits. In particular it ispossible to alternate sequences in which voltage is applied (currentdensity not zero) and relaxation sequences (voltage and current zero);the values of current density used during each anodisation sequence maybe constant or variable, and they may be the same or different from onesequence to another; each anodisation sequence may be of the same or adifferent duration; each relaxation sequence may be of the same or adifferent duration and such durations are independent of the durationsof the anodisation sequences. In that case certain anodisation sequencesmay have current densities of less than 0.01 A/dm².

This variable-mode embodiment of the invention makes it possible toreduce the total duration of the treatment in comparison with theconstant-mode embodiment and also makes it possible to reduce in eachanodisation sequence the value of the current density used.

The process according to the invention makes it possible to provide fordurable and effective passivation of copper in baths of molten fluoridesby virtue of the production of a protective layer which is formedessentially by a mixed fluoride of copper, which is found to have a highrate of covering for the copper substrate, a high level of compactnessin respect of the arrangement of elementary particles, a high level ofadhesion and substantial resistivity. That layer thus prevents anodicdissolution of the copper. Those properties are increasingly marked inproportion to decreasing current density and increasing treatment time.

Those properties are indicated by measuring the leakage current passingthrough the protective layer formed, by virtue of a given voltageapplied across the layer. Generally it is measured, with the part beingimmersed in a conductive bath, for example the passivation bath, byapplying a dc voltage between the part and another immersed electrode.

Thus a copper part passivated in accordance with the prior art by simplybeing dipped in a liquid KF, xHF bath, has a leakage current of 25mA/dm² under a voltage of 5 V. In contrast a part passivated by theprocess according to the invention in the same type of bath has aleakage current which does not exceed 5 mA/dm² under a voltage of 10 Vand usually close to or less than 3 mA/dm² under a voltage of 10 V.

The protective layer is also mechanically strong while in addition it isvery thin so that it does not significantly alter the dimensions or thegeometry of the passivated parts.

The process according to the invention can be used for the passivationof all kinds of copper parts which are subsequently to be used in amedium of molten fluorides or in aqueous solution.

The copper parts passivated by means of the process according to theinvention provide very good resistance to chemical corrosion in allmedia containing fluorides, in particular baths of molten fluorides andmore especially baths containing at least hydrogen fluoride and afluoride of alkali metals or ammonium. Because the protective layer hasgood adhesion and markedly improved mechanical properties, it ispossible to use the passivated parts in a calm or agitated, homogeneousor heterogeneous medium.

However the process finds its particular area of use in the passivationand protection of copper parts, in particular bars for feeding currentto the electrodes which are installed in fluorine electrolysers usingliquid KF, xHF baths as the electrolyte, by virtue of the improvedquality of the layer formed which has good resistance to the bath, thefluorine and the current. The fact that those parts have voltage appliedthereto does not affect their resistance to corrosion.

It is possible to measure the amount of wear of parts which arepassivated in accordance with the process of the invention by immersingthem in the molten bath and subjecting them to an anodic voltage for aweek, as mentioned above, and weighing the part before and after thetreatment. The following results were thus noted on cylindrical discs ofa diameter of 35 mm, with rounded edges, in a bath KF, xHF:

for a part passivated by simply being dipped in accordance with theprior art and subjected to an anodic voltage of 5 V, the leakage currentis 25 mA/dm², the weight loss corresponding to an amount of wear of3mm/year;

for a part passivated in accordance with the process of the inventionsubjected to an anodic voltage of 10 V:

if the leakage current is 3 mA/dm², the weight loss corresponding to anamount of wear of 0.35 mm/year,

if the leakage current is 3.5 mA/dm², the corresponding wear is 0.4mm/year,

if the leakage current is 5 mA/dm², the corresponding wear is less than0.6 mm/year.

The very good quality of the passivation effect produced makes itpossible, when used for the electrolysis of fluorine, to increase theservice life of the copper parts to at least 5 years and to use newelectrolysis cell technologies, in particular supplying the anodes atthe bottom, having regard to the fact that copper parts passivated inaccordance with the process can be immersed and put under voltagewithout problem.

EXAMPLES

The following examples provide non-limitative illustration of differentoperating conditions of the process according to the invention.

EXAMPLE 1

Passivation by means of a current of constant strength.

A disc of copper of type Cu a 1 of a diameter of 35 mm and with a totalsurface area of 0.2 dm² is subjected to an anodic voltage such that thestrength of the current is maintained constant at a value of 3 mA (0.015A/dm²) for a period of 12 hours 30 minutes, with a cathode of steelwhich is identical to the anode, in a bath KF, xHF containing 40.5% byweight of HF, at 95° C.

After treatment the leakage current observed with a voltage of 10 voltsis 3.5 mA/dm².

EXAMPLE 2

Passivation in stages of decreasing anodisation current density,alternately with relaxation periods (variable mode).

The copper disc and the bath are identical to those of Example 1. Thetreatment procedure is as follows:

anodic voltage such that the current strength is maintained at a valueof 2.8 mA (0.014 A/dm²) for a period of 3 hours,

voltage zero (relaxation) for 30 minutes,

anodic voltage such that the current strength is maintained at a valueof 2.8 mA (0.014 A/dm²) for a period of 3 hours,

relaxation for a period of 30 minutes, and

anodic voltage such that the current strength is maintained at a valueof 1 mA (0.005 A/dm²) for a period of 3 hours.

After treatment the leakage current observed with a voltage of 10 V isonly 2.0 mA/dm² while the treatment time is only 10 hours.

EXAMPLE 3

Passivation by means of a current of constant strength in a bath ofanother composition. The disc used is identical to that used inExample 1. The bath is a HF-KF mixture containing 38% by weight of HF at85° C. The copper part is passivated under an anodic current of 3 mA(that is to say 0.015 A/dm²) for about 3 hours 30 minutes.

After treatment the leakage current observed with a voltage of 10 V is 1m/dm², which is revealed by an amount of corrosion of 0.12 mm per year.

EXAMPLE 4

Passivation by means of a current of constant strength which is appliedfor an insufficient time.

This example uses a copper disc, a bath and a temperature which areidentical to those of Example 1. The current strength is maintained at avalue of 0.08 A/dm² for a period of 0.5 hour.

After treatment the leakage current observed is 13 mA/dm², correspondingto a mean amount of wear of 1.5 mm/year. That low value is to becompared to the amount of wear of 3 mm/year for a part which ispassivated by simple dipping. However it results in a reduction incorrosion of the copper, which is still inadequate from the point ofview of the man skilled in the art.

We claim:
 1. A process for the passivating anodisation of copper partsin a liquid KF-xHF medium, where x is about 2, which makes it possibleto produce a mechanically and electrically strong, adherent protectivelayer, with a high rate of covering of the copper substrate, comprisingimmersing said parts in the liquid KF-xHF bath they and subjecting saidimmersed parts to an anodic current of a surface-related density,calculated with respect to the immersed surface of copper to be treated,of lower than 0.1 A/dm².
 2. A process according to claim 1 wherein thesurface-related current density is preferably lower than 0.05 A/dm². 3.A process according to claim 1 or 2, wherein the duraction of thetreatment with a low current density is higher than a limit value.
 4. Aprocess according to claim 1 or 2, wherein the treatment time is longerthan 0.5 hour.
 5. A process according to claim 1, wherein the anodiccurrent density is maintained at a constant value during the treatmenttime.
 6. A process according to claim 1 or 2 wherein the anodic currentdensity is of a variable value in the course of the treatment.
 7. Aprocess according to claim 6 comprising alternating anodisationsequences with a current density which is not zero and relxationsequences with a zero current density.
 8. A layer for protecting copperparts, produced by the process according to claim 1 or 2, consistingessentially of a mixed compact copper fluoride having a high rate ofcovering of the substrate.
 9. A layer for protecting copper parts,produced by the process according to claim 1 or 2, wherein the leakagecurrent measured across said layer under a voltage of 10 V is less than5mA/dm².
 10. A process according to claim 6, wherein the value of thecurrent density decreases from one sequence to the next.
 11. A processaccording to claim 4, wherein the anodic current density is maintainedat a constant value during the treatment time.
 12. A process accordingto claim 4, wherein said treatment time is between 2 and 4 hours.
 13. Alayer for protecting copper parts according to claim 9, wherein saidleakage is less than 3 mA/dm².